Method and apparatus for contacting vapors with fluidized solids



1955 c. A. REHBEIN 2,700,541

\ METHOD AND APPARATUS FOR CONTACTING VAPORS WITH FLUIDIZED souos FiledFeb. 12, 1952 Y Y 2 Sheets-Sheet l v- PRODUCT CYCLONE SEPARATOIZS a Y rh4 5 L5 r 1 REGENERATOR STEAM 1A m STEAM STUPPN ZONE.

lnvn+br= charms A. Rzhbzin Jan. 25, 1955 c. A. REHBEIN 2,700,641 METHODAND APPARATUS FOR CONTACTING VAPORS WITH FLUIDIZED SOLIDS Filed Feb. 12,1952 2 Sheets-Sheet 2 lnvznTor: Charles A. Rch al'n METHOD AND APPARATUSFOR VAPORS WITH FLUIDIZED SOLIDS Application February 12, 1952, SerialNo. 271,097 2 Claims. (Cl. 196-52) CONTACTING This application is acontinuation-in-part of my copending application, Serial No. 593,625,filed May 14, 1945, now U. S. Patent No. 2,606,863.

The invention relates to an improved apparatus and method for contactingvapors with fluidized finely divided solids, and, in particular, to thecatalytic cracking of hydrocarbon oils with fluidized cracking catalystand the regeneration of the spent cracking catalyst by burning offcarbonaceous deposits.

Fluidized catalyst catalytic cracking reactors, whether used to convertan oil or to react off carbonaceous deposits from the fouled catalyst,may be divided into two distinct classes which are known as up-flowreactors and down-flow reactors, respectively. In both types, a vapor,e. g. oil vapor, and finely divided solid are introduced at or near thebottom of a fluidized bed of the solid in the reactor.

In the up-flow type of reactor the vessel is so con.- structed that anamount of solid equivalent to that intro duced near the bottom iscontinuously withdrawn as a dilute suspension (dilute phase) with theeffluent vapors at the top. This suspension, carrying, for example, 30tons of the solid per minute, is passed to external separating means andthe separated solid is then handled in an appropriate manner and finallyrecycled to the bottom of the reactor.

In the downflow type of reactor carry-over of the solid in suspension inthe efliuent vapors is held at a minimum and is, consequently,inconsequential. The amount of material carried out of the system inthis manner is generally too small to Warrant attempts to recover it. Inthe down-flow type of reactor; the solid is withdrawn by gravity fromthe dense phase bed within the reactor. The down-flow type of reactorhas certain advantages. The present invention relates to an improvementin the de sign and operation of reactors of this type.

In down-flow reactors, the dense phase catalyst withdrawn from thereactor by gravity would normally contain a considerable amount of thereactant vapors adsorbed on the particles and occluded in the movingmass. In order to avoid a substantial loss of this material and also forother reasons, it is necessary in practice to remove as much of thisadsorbed and occluded material as possible. This is done by a so-calledstripping step in which the stream of solid being withdrawn is flushedwith a stripping vapor such, for example, as steam. In spite ofconsiderable work and considerable practical experience, little is knownof the fundamentals of this stripping step. Various designs andalterations have been tried but none have materially improved theefiiciency of this importantstep by any substantial extent. So-calledexternal strippers have been built, but this more costly arrangement hasnot shown any material advantage over the more conventional internalstripping arrangements, i. e., arrangements wherein the stripping takesplace in a so-called stripping zone within the reactor vessel. presentinvention relates to apparatus wherein the less costly internalstripping is carried out.

Internal stripping is effected in a part of the reaction vessel zonedfrom the. remaining part of the vessel by a partition or other suitable.structure. In one common type of structure, the incoming vapors andsolids are introduced intov the fluid bed through a central tankarrangement provided with a perforated top and the strip ping iseffected in the. annular space around the side of the tank. In anotherarrangement, a vertical partition is extended from the bottom of thevessel upward into the The States Patent fluidized bed of solid. Thatpart of the bed to one side of the partition, and also all of the bedabove the level of the top of the partition, is utilized in effectingthe reaction and is known as the reaction section or reaction zone, andthat part on the other side of the partition and below the level of thetop of the partition constitutes the stripping zone. It is obvious thatin both of these cases, the volume of the stripping zone, whatever itmay be, detracts from the volume of the vessel which otherwise could beutilized for effecting the desired reaction. It is, therefore, thedesire to maintain the stripping zone as small as possible and, at thesame time to utilize the alloted space as efllciently as possible.

hile, as pointed out, little is known about the fundamentals of thestripping operation, it is known that, other conditions being equal, thestripping efliciency increases as the stripping time is increased. Thus,if the stream of the solid is retained in the stripping zone for alonger time while passing a stripping gas through it, it is moreeffectively stripped. The residence time of the solid in the strippingzone at a given flow of so many tons per hour of the solid depends uponthe volume of the stripping zone and upon the density of the fluidizedsolid in the stripping zone. The maximum residence time is, therefore,obtained with a stripping zone of given volume when the stripping zoneis retained completely full of the fluidized solid and the fluidizedsolid is at its highest applicable density. This condition exists in thedesigns just described since the stripping zone is below the upper levelof the fluidized bed and is in open communication with the main mass offluidized solid. It would, therefore, be expected that these previouslyused arrangements should provide the maximum efiiciency for a strippingzone of any given volume.

In my copending application, referred to above, I have shown thatcontrary to expectation, the stripping efficiency can be appreciablyincreased by a relatively simple expedient which results in decreasingthe residence time of the solid in the stripping zone. This improvementwhich has been applied in commercial practice, and has demonstrated itseficiency is obtained by holding the level of the dense phase (pseudoliquid phase) in the stripping zone below the top of the partitionthereby causing the fluidized solid to flow over the top of thepartition as water over a dam and fall or rain down to the lower levelin the stripping section. In this case, for some unexplained reason, thestripping efliciency is considerably improved even though the spacealloted to the stripping zone is not filled with dense phase solid.

In the arrangement shown and described in the said copendingapplication, the stripping zone in which the low dense phase level ismaintained is formed by a centrally located open top cylindricalpartition which extends by necessity to exactly the top level of thefluidized bed constituting the reaction zone. The method is, however,not restricted to this particular location of the stripping zone.

While the centrally located stripping zone illustrated in my copendingapplication can be used and has been applied commercially, thisparticular arrangement has a material drawback. It is found in practicethat it is virtually impossible with this arrangement to effect gooddistribution of the reactant vapors over the cross-section of the largeannular reaction zone without resorting to a high and costly pressuredrop through the grid arrangement. A much more satisfactory arrangementis to construct the stripping zone as an annular zone surrounding thereaction zone. Another more satisfactory arrangement is that in whichthe partition extends as a chord across the horizontal cross-section ofthe reaction vessel, thus creating a small stripping zone between theperiphery of the vessel and the partition. These more suitablearrangements are applied in the method and apparatus in the presentinvention.

In the process and apparatus of my present invention, the considerableimprovement described in my mentioned copending application is obtainedwith the better arrangement of the stripping zone just described. Ihave, however, now found that this arrangement allows further importantimprovements in the process to be realized, provided that a furthersimple modification of the design is made. In order to explain the causeand nature of these lmportant further improvements, it is necessary toagain refer to the limitations hitherto existing in the reactors of thetype in question and to the reasons for these limitations. As mentionedabove, in the downflow type of reactor, the solid is withdrawn bygravity from the fluidized bed in the reactor and carry-over of solidwith the eflluent vapor is undesired and held at a minimum. Thecarry-over of any appreciable amount of the solid is prevented bypassing the vapors through a cyclonetype separator which may be ofsingle stage or multi-stage construction. This separator, for verypractical reasons, is within the reactor vessel and is mounted near thetop of the vessel. The gas or vapor carrying some solid in suspensionenters a suitable opening near the top of the separator and, aftertaking a spiral path downward, passes upward through a central channel.The solid which is thrown to the wall of the separator during thedownward spiral flow collects in the bottom of the separator vessel. Inorder to return the separated solid to the main mass of the solid and toprevent the separator from becoming quickly plugged with solid, thepowder collected at the bottom of the separator vessel is withdrawn bygravity through a so-called dip leg. It is essential that provision bemade to prevent gas or vapor from passing up through this dip leg. Itis, therefore, essential that the bottom of the dip leg be sealed insome manner against any appreciable entrance of gas and this is usuallydone by utilizing the liquid properties of the main mass of fluidizedsolid. Thus, the dip leg is caused to dip into the fluidized bed. Inorder to obtain sufficient head to cause the separated solid to flowdown the dip leg back into the fluidized bed, a dip leg of at leastabout 8 feet is necessary. (The static head caused by a column of theconventional fluidized cracking catalyst is about 0.24 pound/ foot oflength.) With this usual arrangement, the bottom of the cycloneseparator must, therefore, be at least about 8 feet above the level ofthe fluidized bed. When the height of the cyclone separator itself(which is appreciable and may be, for example, 15 feet) is added tothis, it is seen that the reactor vessel must extend above the fluidizedbed level by a very substantial distance, e. g., 20-30 feet. In otherwords, in downflow reactors of the designs hitherto used, it has beennecessary to maintain the level of the fluidized bed well below the topof the reactor vessel. In a cylindrical reactor vessel, there is,therefore, a large unused volume which, if it could be utilized, wouldaccommodate a much larger fluidized bed.

This limitation is removed in the process of the invention by returningthe separated catalyst from the cyclone separator to the fluidized bedin the stripping zone where, as explained, a low level is maintained. Inorder to do this, it is necessary to pass the dip leg of the cycloneseparator through the partition which separates the stripping zone fromthe reaction zone. It then becomes possible to greatly extend the heightof the partition to within a short distance of the inlet openings of thecyclone separator. The level of the fluidized bed in the reaction zoneis, therefore, increased and this large additional volume becomesutilizable.

It will be seen from the above that according to the present invention adownflow type of reactor is used to obtain the known advantages of thistype of reactor. The stripping zone is within the reactor vessel and islocated at the outside horizontal periphery of the reaction zone toavoid the difliculties in distributing the reactant hitherto encounteredwith other designs. The cyclone dip leg is caused to pass through thepartition which separates the reaction zone from the contiguousstripping zone and to discharge into the fluidized bed in the strippingzone. The fluidized bed in the stripping zone is retained at a low levelthereby allowing the separated solid to pass through the dip leg intothe stripping zone and, at the same time, affording the increasedefliciency of the stripping step. The height of the partition isincreased materially thereby increasing materially the height of thefluidized bed in the reaction zone and the amount of solid thalt areactor of given external dimensions can profitably uti ize.

The advantages which can be gained by applying the improvements of myinvention are very substantial and are as follows: the etficiency of thestripping step is materially increased as shown in my mentionedcopending application. Adequate distribution of the reactant vapor overthe cross-section of the fluidized reaction zone is obtained with a lowpressure drop. For a reactor of given throughput capacity, the heightand/or diameter may be materially decreased. This not only represents alarge saving in capital cost for the vesse itself, but also in thesupporting structure. On the other hand, with a given reactor thethroughput capacity may be increased markedly, in some cases twofold.Moreover, by decreasing markedly the vapor space above the fluidized bedin the reactor, the secondary reactions which normally take place inthis space are materially reduced. This is particularly advantageouswhen the apparatus is employed for regeneration of spent catalyst sinceit materially reduces the problem caused by the phenomenon known asafter burning. After burning is a secondary combustion which is found totake place in the space above the fluidized bed; it is known to be mostdetrimental to the catalyst.

The principles of the invention will be more clearly understood from thediagrammatic illustrations in the accompanying drawing.

Referring to the drawing, Figure I is a flow diagram in which areindicated the essential relative levels of the fluidized beds in thereaction zones and stripping zones and the positions of the cycloneseparators and their dip legs. Figures 11 and III are diagrammaticillustrations of alternative ways of partitioning the vessels to providesuitable stripping zones and reaction zones.

Referring to the drawing, Figure I, a reaction vessel 1 and regenerationvessel 2 are diagrammatically indicated. These vessels may be ofdifferent size and/ or design. The reaction zones are separated in eachcase from their corresponding stripping zones by a partition 3 whichextends across the cross-sections of the vessels as chords. Thus, asillustrated in Figure 11, the smaller section on the lefthand is thestripping zone and the larger section on the right is the reaction zone.An alternative, and in some respects superior arrangement, isillustrated diagrammatically in Figure III. Here the reaction zone iscentrally located and the stripping zone is in the form of an annularzone surrounding the reaction zone.

As illustrated in Figure I, the cyclone separators 4 are built withinthe vessel and are placed at the top. Twostage cyclone separators of theconventional design are diagrammatically indicated. Vapors carryingsuspended solids enter the first stage of the cyclone separators bytangential inlets 5 and, after passing through the two stages, the vaporis discharged by line 6 substantially free of suspended particles. Thepowder separated in the two-stages in the cyclones collects in theconical section 7 of the cyclones and is continuously withdrawn bygravity through dip leg 8. Dip leg 8 passes through the partition 3 anddischarges into the stripping zone at a low level. As previouslyindicated, this dip leg must be sufficiently long to cause the separatedpowder to flow by gravity; from practical experience the minimum lengthis known to be about 8 feet.

In reactors of the conventional design, i. e., with the cyclone dip legdischarging in the reacton zone, the maxi' mum level of the fluidizedbed of catalyst in the reaction zone would be approximately as indicatedby the broken lines A, and the partitions 3 would end at this level. Bypassing the cyclone dip leg 8 through the partition nto the strippingzone and holding the level of the fluidized bed in the stripping zone ata low level (below A), the partition 3 can be extended at least 5 feetand, if desired, to within two or three feet of the cyclone inlets.Thus, the level of the fluidized bed can be raised until it actuallycovers the lower parts of the cyclone separators. The spent catalystcontinuously overflows the top of the partition and in doing so rainsdown to the flu1d1zed bed in the lower section of the stripping zone. Asexplained above, this feature, proves the stripping efficiency.fluidized catalyst bed that therefore is indicated in the drawing by thecross hatched section of the fluidized bed above the broken line A.

The operation of the system illustrated is otherwise conventional. gasis introduced near the bottom of the stripping sections by lines 9 and10. Stripped catalyst is withdrawn from the respective'vessels bystandpipes 11 and 12 and, after passing through control valves 13 and14, are picked up and carried to the reaction section of the oppositevessel by air and oil introduced by lines 15 and 16, respectively. Themixtures are discharged over the cross-sectipns of the respectivereaction zones by means of gas distribution Thus, steam or othersuitable stripping means such as grids, indicated in the drawing by thebroken lines 17 and 18.

The path of the main mass of catalyst starting at the control valve 14is, therefore, as follows: hot, freshly regenerated catalyst is pickedup by oil and carried by line 19 as a dilute suspension into the densephase bed of catalyst in the reaction zone. After becoming partiallyspent in the reaction zone, the powdered catalyst overflows the top ofthe partition 3 and, after falling as a dilute phase countercurrent touprising vapors of stripping gas, it collects in a low level, densephase bed in the stripping zone. Here it is stripped of adsorbedhydrocarbons by steam introduced by line 9. The spent and strippedcatalyst descends as a dense phase of gravity through the standpipe l1and control valve 13 and is picked up by air and carried as a dilutephase in line 20 to the dense phase bed in the regenerator. After beingregenerated in the dense bed, the catalyst overflows the top of thepartition and falls as a dilute phase to the dense phase low level bedin the stripping zone of the regenerator. Inert gas or steam isintroduced at the bottom of the stripping zone by line 10. Theregenerated catalyst stripped of occluded oxygen is withdrawn as a densephase by the standpipe 12 to complete the cycle.

A minor amount of the catalyst is thrown up above the level of the densecatalyst phase in the respective reaction zones. Most of this catalystfalls back into the dense phase bed and overflows the partition asdescribed. However, some remains suspended in the dilute phase above thecatalyst bed. This material is separated from the vapors by the cycloneseparators and is returned to the catalyst mass in the dense phase bedin the stripping zone by means of the dip leg 8.

In operation it is essential that the fluidized bed of catalyst in thestripping zone be maintained at a low level which is considerably belowthe level of the top of the partition. If for any reason the level inthis zone should be allowed to rise to the top of the partition, thecyclones would cease to function and the process would becomeinoperative. In practice it is, therefore, essential that suitablesafeguards be provided for automatically controlling the level of thefluidized bed in the stripping zone. A suitable control is described inmy abovementioned copending application and in Figure IV of theaccompanying drawing which shows diagrammatically an enlarged section ofthe right hand side of the vessel 2 of Figure I. As described, thecatalyst level in the reaction zone to the left of the partition 3 ishigher than normal and equal to the level of the top of the partition,whereas the bed level in the stripping section to the right of thepartition 3 is held low. This low bed level is adjusted and thenmaintained automatically by a dilferential pressure controllerinstrument which is connected to be responsive to difierences inpressure measured by suitable pressure bulbs 21 and 22 situated wellabove and below the desired catalyst level, respectively, and is in turnconnected to control a suitable valve such as a slide valve in thedischarge line in response to changes in the differential pressurebetween these points. The instrument is connected in such a manner thatwhen the differential pressure falls below a selected pointcorresponding to the desired level, the valve is moved to a more nearlyclosed position, and vice versa.

In the plant indicated diagrammatically in the drawing, it will be notedthat both the reactor and regenerator operate on the same principle. Inthis particular circumstance, the control problem is greatly sirnplifiedsince the level of the fluidized catalyst in the stripping zones is afunction of the catalyst inventory. Thus, the desired level isestablished when the catalyst is charged to the plant (with air via line20). A small quantity of makeup catalyst is then added from time to timeto maintain the inventory and the desired level.

I claim as my invention:

1. In the catalytic cracking of a hydrocarbon oil with a finely dividedsolid catalyst which is continuously recirculated through a reactionvessel and a separate regeneration vessel and wherein the finely dividedsolid catalyst which is continuously supplied to the regeneration vesselis withdrawn from the reaction vessel by gravity flow from a fluidizedbed of the catalyst in the reaction vessel, the improved method ofoperation which comprises maintaining a high level of fluidized catalystbed in the reaction zone, maintaining a low level of fluidized catalystin a contiguous stripping zone by automatic control of the withdrawal ofpowder from said stripping zone, separating suspended catalyst particlesfrom the mixed vaporous effluent above said contiguous stripping andreaction zones, passing the thus separated catalyst by gravity flowdirectly through the upper portion of the catalyst bed in the reactionzone in indirect contact into said low level bed of fluidized catalystin said stripping zone, the level of fluidized bed in said reaction zonebeing above the maximum level which allows said separated catalyst toflow by gravity directly to said reaction zone and the level offluidized bed in said stripping zone being below said maximum level.

2. Apparatus for contacting a gaseous fluid with a continuously renewedbed of finely divided solid in fluidized condition which comprises incombination, a vertically disposed cylindrical vessel with top andbottom closures, a cluster of cyclone-type dust separatorsconcentrically located within said vessel near the top thereof, outletfor dust free fluid from said dust separators, said outlet passingthrough said top closure, a substantially vertical partition dividingthe major volume of said vessel into at least two separate side by sidefluid-tight compartments which communicate only above the top of saidpartition, said partition furthermore dividing the horizontalcross-section of said vessel throughout the greater part of its lengthinto a major and more centrally located reaction area and a minorstripping area located near the outer Wall of the vessel, said partitionfurthermore extending upward from the effective bottom of said vessel toa point well above the center of the vessel but below the inlet ports ofsaid cyclone separators, a line for removing separated powder by gravityflow from said cyclone type separators, said line depending from saidseparators and extending through said partition to discharge in the saidminor stripping area at a point below the top of said partition, fluiddistribution means located near the bottom of said compartments, inletand outlet lines communicating with said major reaction area and minorstripping area, respectively, below said fluid distribution means, andmeans for automatically controlling the rate of withdrawal of fluidizedpowder through said outlet line to maintain a level of fluidized bed insaid stripping area below the level of the top of said partition.

References Cited in the file of this patent UNITED STATES PATENTS2,327,175 Conn Aug. 17, 1943 2,514,288 Nicholson July 4, 1950 2,541,186Anderson Feb. 13, 1951 2,650,155 Medlin Aug. 25, 1953

1. IN THE CATALYTIC CRACKING OF A HYDROCARBON OIL WITH A FINELY DIVIDEDSOLID CATALYST WHICH IS CONTINUOUSLY RECIRCULATED THROUGH A REACTIONVESSEL AND A SEPARATE REGENERATION VESSEL AND WHEREIN THE FINELY DIVIDEDSOLID CATALYST WHICH IS CONTINUOUSLY SUPPLIED TO THE REGENERATION VESSELIS WITHDRAWN FROM THE REACTION VESSEL BY GRAVITY FLOW FROM A FLUIDIZEDBED OF THE CATALYST IN THE REACTION VESSEL, THE IMPROVED METHOD OFOPERATION WHICH COMPRISES MAINTAINING A HIGH LEVEL OF FLUIDIZED CATALYSTBED IN THE REACTION ZONE, MAINTAINING A LOW LEVEL OF FLUIDIZED CATALYSTIN A CONTIGUOUS STRIPPING ZONE BY AUTOMATIC CONTROL OF THE WITHDRAWAL OFPOWER FROM SAID STRIPPING ZONE, SEPARATING SUSPENDED CATALYST PARTICLESFROM THE MIXED VAPOROUS EFFLUENT ABOVE SAID CONTIGUOUS STRIPPING ANDREACTION ZONES, PASSING THE THUS SEPARATED CATALYST BY GRAVITY FLOWDIRECTLY